Fascioloides magna is up to 100 mm long, 2–4.5 mm thick, 11–26 mm wide, and oval; it is distinguished from Fasciola spp by its large size and lack of an anterior projecting cone. It is found in domestic and wild ruminants; deer are the reservoir host. The life cycle resembles that of Fasciola spp.
Although flukes will mature in cattle, the intense encapsulation response forms a closed cyst, so eggs rarely pass out of the animal. Pathogenicity is low, and losses are confined primarily to liver condemnations. In sheep and goats, encapsulations do not occur, and the parasites migrate in the liver and other organs, causing tremendous damage. A few parasites can cause death due to extensive migrations. Infection with F magna appears to be rare in alpacas and llamas, in which the response mirrors that seen in cattle. In deer, there is little tissue reaction, and the parasites are enclosed in thin, fibrous cysts that communicate with bile ducts. Histologically, infected livers of all species show black, tortuous tracts formed by migrations of young flukes.
While the eggs of F magna resemble those of Fasciola hepatica, this is of limited use; eggs usually are not passed by cattle and sheep and probably not by alpacas and llamas. Recovery of the parasites at necropsy as well as differentiation of F hepatica is necessary for definitive diagnosis. When domestic ruminants and deer share the same grazing, the presence of disease due to F magna should be kept in mind. Mixed infections with F hepatica are seen in cattle.
Oxyclozanide has been reported to be effective against F magna in white-tailed deer, and triclabendazole has been used in captive and free-ranging red deer. Rafoxanide has been used successfully against natural infections in cattle. Albendazole (7.5 mg/kg), clorsulon (15 mg/kg), and closantel (15 mg/kg) have shown efficacy against this fluke in sheep. Currently, no products are approved for use against this fluke in the USA. Deer are required for completion of the life cycle; if they can be excluded from the areas grazed by cattle and sheep, control may be effected. Control of the intermediate host (lymnaeid snails) may be possible once it has been identified in a region and the nature of its habitat examined. However, environmental concerns are the same as those for F hepatica.